This invention relates in general to fasteners, and more invention relates in general to fasteners, and more particularly to an improved fastener mandrel and method for securing the fastener mandrel with relation to the fastener sleeve.
A blind rivet is generally composed of two pieces. The first piece is the mandrel, which is composed of a cylindrical stem having an enlarged head at one end. The second piece is the sleeve, a generally tubular member surrounding a portion of the mandrel stem and abutting the mandrel head, with an outwardly protruding flange at the end opposite the mandrel head.
In use, such a fastener is typically placed in the pulling head of the powered fastener installation device and directed to the workpiece, which commonly consists of a plurality of members to be fastened. In other instances, the fastener is placed in the workpiece and the powered fastener installation device is then applied to it. In either case, the stem of the fastener is gripped by the jaws of the fastener installation device pulling head, which is then operated by hydraulic pressure to clamp the jaws radially about the fastener stem and pull the stem rearward away from the workpiece. At the same time, a reactive force is applied to the sleeve flange, urging it forward against the workpiece. The tension on the stem pulls the stem head against the sleeve, thereby upsetting the sleeve.
Once the sleeve has been upset, the sleeve flange and mandrel head clamp the members of the workpiece together. After the fastener sleeve is upset in the manner described above, and the workpiece members are clinched together, resistance to the stem movement, and thus tension in the stem, increases under the continued application of the pulling force.
Continued application of a compression load on the sleeve flange causes the bore of the sleeve to constrict as material flows inward toward the mandrel stem. The sleeve material that flows inward into the locking grooves creates an interference fit that inhibits withdrawal of the mandrel from the sleeve.
The stem is provided with a locking section having grooves thereon. The locking grooves may take the form of one or more annular grooves, as shown, for example, in U.S. Pat. No. 3,230,818 to G. Siebol. It has been found, however, that a thread-like spiral locking groove is preferable to annular grooves. Although a locking section having a spiral locking groove may not provide as high an installed mandrel retention load (the force required to pull an installed mandrel out of its sleeve) as one having annular grooves, the installed mandrel retention load will be uniform along the length of the spiral locking groove. On the other hand, in a locking section having annular grooves, the installed mandrel retention load depends on where the locking section is located when the sleeve is deformed to effect mandrel retention. Obviously, the material will not flow into the ridges between locking grooves. The annular ridges can, therefore, provide an area in which there is little if any resistance to withdrawal of the mandrel. In a locking section with a spiral locking groove, those areas of minimal resistance are distributed lengthwise along the locking section.
Spiral locking grooves offer an additional advantage over annular grooves. The edges of a spiral locking groove are significantly less likely to abrade the sleeve bore than the edges of an annular locking groove. Annular locking grooves may tend to break small chips of material off the inside of the sleeve during installation of the fastener. These chips may accumulate in the fastener installation device and, eventually, clog the device. The material buildup, tool damage and resulting downtime can be quite expensive.
It is conventional in the art to provide the stem with a weakened section, termed a "breakneck" groove. The mandrel stem is designed to fracture at the breakneck groove when the tension reaches a predetermined maximum load during installation, whereupon the gripping section of the stem separates from the remainder of the stem disposed within the sleeve. As is commonly known in the art, the breakneck groove is formed in a two-step rolling process. First, the groove is notched into the mandrel blank. This process creates bulges or ridges of material displaced on either side of the groove which present relatively sharp corners. If these ridges were not removed, they would scoop material from the sleeve bore during installation, creating chips of material that may clog the fastener installation device, as discussed above. Accordingly, the stem is rolled so that the area next to the breakneck groove is rolled down and the groove is rolled almost completely shut, thereby reducing any sharp corners that could catch the sleeve bore.
The prior art has long sought to improve the installed mandrel retention load of fasteners. These efforts have focused on ways to improve the flow of material from the sleeve into the grooves in the mandrel stem locking section. One approach has been to apply axial force to the fastener sleeve flange with a pulling head nosepiece having a sharp protruding configuration which acts to stake material from the sleeve into the locking grooves. An example of this approach is the Cherry T Rivet..RTM. Use of such an arrangement, however, may result in the production of tiny chips of material, which are broken from the sleeve during displacement of the sleeve material by the pulling head nosepiece. As discussed above, these tiny chips are quite detrimental. In addition, such sharp protruding nosepieces are relatively expensive, and their use renders the fastener installation process sensitive to wear or damage to the nosepiece. An incorrect, worn or broken sharp protruding nosepiece will preclude obtaining the proper interference fit between the sleeve and mandrel during installation of the fastener. The sharp protruding nosepiece tends to wear and break much more quickly than a planar nosepiece, which causes problems both due to inadequate fastener installation, increased cost of nosepiece replacement, and increased downtime. Because the operators of such equipment may be relatively unskilled, the worn or broken nosepiece may go unnoticed. Thus, a large number of fasteners may be improperly installed before the defective nosepiece is discovered. This can lead to weak joints or complete "pop-out" of the mandrels, in either case resulting in expensive repair of the workpiece with concomitant downtime.
The traditional problems with specially configured fastener installation tool nosepieces have been exacerbated by the recent application of powered fastener installation devices to automated assembly. These robot-mounted devices require a sturdy and simple design of the nosepiece so as to reduce the potential for error and downtime. The high installation volume and inability of the automated power fastener installation device to detect defects in the nosepiece render this problem critical.
The problems associated with the sharp protruding nosepiece have been reduced by use of a special fastener sleeve configuration, as disclosed in my co-pending patent application, Ser. No. 217,318, entitled "Fastener With Integral Locking Means," the disclosure of which is hereby incorporated by reference. Nevertheless, the need remains to improve the installed mandrel retention load of such fasteners. Accordingly, there exists a need for enhancing the interference between an installed fastener sleeve and mandrel having a spiral lock groove.